1. Field
The present disclosure relates generally to methods for forming activated carbon, and more specifically to physical activation of resin-based carbon using carbon dioxide.
2. Technical Background
Energy storage devices such as ultracapacitors may be used in a variety of applications such as where a discrete power pulse is required. Example applications range from cell phones to hybrid vehicles. Ultracapacitors also known as electrochemical double layer capacitors (EDLCs) have emerged as an alternative or compliment to batteries in applications that require high power, long shelf life, and/or long cycle life. Ultracapacitors typically comprise a porous separator and an organic electrolyte sandwiched between a pair of carbon-based electrodes. The energy storage is achieved by separating and storing electrical charge in the electrochemical double layers that are created at the interfaces between the electrodes and the electrolyte. Important characteristics of these devices are the energy density and power density that they can provide, which are both largely determined by the properties of the carbon that is incorporated into the electrodes.
Carbon-based electrodes suitable for incorporation into energy storage devices are known. Activated carbon is widely used as a porous material in ultracapacitors due to its large surface area, electronic conductivity, ionic capacitance, chemical stability, and/or low cost. Activated carbon can be made from synthetic precursor materials or natural precursor materials, such as coals, coconut shells, and biomass. With both synthetic and natural precursors, the activated carbon can be formed by first carbonizing the precursor and then activating the intermediate product. The activation can comprise physical (e.g., steam) or chemical activation at elevated temperatures to increase the porosity and hence the surface area of the carbon.
Both physical and chemical activation processes typically involve large thermal budgets to heat and react the carbonized material with the activating agent. In the case of chemical activation, corrosive by-products can be formed when a carbonized material is heated and reacted with an activating agent such as KOH. Additionally, phase changes that may occur during the heating and reacting of the carbonized material and chemical activating agent can result in undesired agglomeration of the mixture during processing. These drawbacks can add complexity and cost to the overall process, particularly for reactions that are carried out at elevated temperatures for extended periods of time.
Significant issues have been reported when caustic materials, such as KOH, are used for the chemical activation of carbon. For example, when rotary kilns are used in carbon activation, it is often required that the feedstock undergoes calcination and/or drying and/or dehydration prior to treatment at activation temperatures. Agglomeration tends to pose significant issues, such as increased process complexity and/or cost, in continuous processes, for instance, processes employing screw kneaders. As a means to avoid agglomeration issues, other technologies such as roller hearths, have been employed wherein trays are loaded with activation mix material and passed through a multiple zone tunnel furnace. Such furnaces may be costly in operation and may have limited throughput since only one tray level is passed through the furnace at a time. The furnace width is also limiting factor for roller hearths on throughput since roller length spanning across the furnace is limited by material availability and strength at service temperature.
Accordingly, it would be advantageous to provide activated carbon materials and processes for forming activated carbon materials using a more economical activation route while also minimizing the technical issues of corrosion and/or agglomeration. The resulting activated carbon materials can possess a high surface area to volume ratio and minimal reactivity, particularly with the organic electrolyte at elevated voltages, and can be used to form carbon-based electrodes that enable efficient, long-life and high energy density devices.